Selective production of para-xylene

ABSTRACT

A catalytic process is provided for the selective production of para-xylene by contacting, under conversion conditions, a charge stock containing, as a major reactant, at least one hydrocarbon selected from the group consisting of toluene, a C 3  -C 10  olefin and mixtures of the foregoing with a catalyst comprising a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12, a constraint index, as hereinafter defined, within the approximate range of 1 to 12 and having combined therewith boron in an amount of at least about 0.2 percent by weight.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of copending application Ser. No. 638,864, filed Dec. 8, 1975, now U.S. Pat. No. 4,029,716.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

This invention relates to a process for converting certain hydrocarbons to a high yield of para-xylene utilizing a boron-containing crystalline aluminosilicate zeolite catalyst.

2. Description of the Prior Art.

The disproportionation of aromatic hydrocarbons in the presence of zeolite catalysts has been described by Grandio et al. in the Oil and Gas Journal, Vol. 69, No. 48 (1971).

U.S. Pat. Nos. 3,126,422; 3,413,374; 3,598,878; 3,598,879 and 3,607,961 show vapor-phase disproportionation of toluene over various catalysts.

Alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units is described in U.S. Pat. Nos. 2,290,607. 3,251,897 describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. Nos. 3,751,504 and 3,751,506 describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g. benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.

In these prior art processes, the xylene product produced has the equilibrium composition of approximately 24 percent para, 54 percent of meta and 22 percent of ortho.

The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200 to 275° C. with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50 percent of the xylene product mixture, being observed at 225° C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in the production of para- and ortho-xylene.

While the above-noted prior art is considered of interest in connection with the subject matter of the present invention, the conversion process described herein, utilizing a catalyst comprising a composite of a crystalline aluminosilicate zeolite and boron oxide, which zeolite has a silica/alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12, has not, insofar as is known been heretofore described.

Of the xylene isomers, i.e. ortho, meta and paraxylene, meta-xylene is the least desired product, with ortho and para-xylene being the more desired products. Para-xylene is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers, such as "Dacron". Mixtures of xylene isomers, either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have previously been separated by expensive superfraction and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.

SUMMARY OF THE INVENTION

In accordance with the present invention, there has been discovered a process for the selective production of paraxylene by contacting, under conversion conditions, a charge stock containing as a major reactant, at least one hydrocarbon selected from the group consisting of toluene, a C₃ -C₁₀ olefin, and mixtures thereof with one another with a catalyst comprising a crystalline aluminosilicate zeolite and boron. The crystalline aluminosilicate zeolite is essentially characterized by a silica to alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12.

The present process comprises conversion of the specified hydrocarbons to yield xylenes in which the proportion of para-xylene isomer is substantially in excess of its normal equilibrium concentration and preferably in excess of 50 weight percent of the xylene product produced in the presence of the specified catalyst at a temperature between about 250° and about 800° C. at a pressure between about atmospheric and about 1000 psig utilizing a feed weight hourly space velocity (WHSV) between about 0.1 and about 2000. The latter WHSV is based upon the weight of catalyst composition, i.e. total weight of active catalyst and binder therefor. The effluent is separated and distilled to remove the desired products, e.g. para-xylene and ureacted product is recycled for further reaction.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The charge stock utilized in the process of this invention contains as a major reactant, at least one hydrocarbon, which can be toluene, a C₃ -C₁₀ olefin, or mixtures thereof with one another.

Typical of the processes contemplated herein are the disporoportionation of toluene to benzene and xylenes, wherein the proportion of para-xylene obtained is greatly in excess of its normal equilibrium concentration. Such process is effectively carried out at a temperature of between about 400 and about 750° C. at a pressure of between about 1 atmosphere and about 1000 psig utilizing a weight hourly space velocity of between about 1 and about 20.

Another process involves the methylation of toluene by reaction of the latter with a methylating agent, preferably methanol, at a temperature between about 250° and about 750° C. and preferably between about 400° and about 600° C. The reaction generally takes place at atmospheric pressure, but the pressure may be within the approximate range of 1 atmosphere to 1000 psig. The molar ratio of methylating agent is generally between about 0.05 and about 5. When methanol is employed as the methylating agent a suitable molar ratio of methanol to toluene has been found to be approximately 0.1-2 moles of methanol per mole of toluene. With the use of other methylating agents, such as methylchloride, methylbromide, dimethylether, methyl carbonate, light olefins, or dimethylsulfide, the molar ratio of methylating agent to toluene may vary within the aforenoted range. Reaction is suitably accomplished utilizing a weight hourly space velocity of between about 1 and about 2000 and preferably between about 5 and about 1500. The reaction product consisting predominantly of para-xylene or a mixture or para- and ortho-xylene, together with comparatively smaller amounts of meta-xylene may be separated by any suitable means, such as by passing the same through a water condenser and subsequently passing the organic phase through a column in which chromatographic separation of the xylene isomers is accomplished.

Another charge stock suitable for use in the process of the invention, is a stream high in C₃ -C₁₀ olefin content. Thus, propylene, butenes, pentenes, hexenes, dienes such as butadiene, pentadienes, cycloolefins such as cyclopentene and cyclohexene, alkyl-substituted cycloolefins such as ethyl cyclopentene, cyclopentadiene and cyclohexadiene can be effectively converted to a high yield of para-xylene utilizing the described catalyst comprising a composite of a specified crystalline aluminosilicate zeolite and boron. Conversion utilizing such olefin feed is carried out at a temperature within the approximate range of 300° to 700° C., a pressure between atmospheric and 1500 psig employing a weight hourly space velocity between about 1 and about 1000. As sources of the olefin reactant either substantially pure streams of the C₃ -C₁₀ olefin may be employed or refinery or chemical streams high in such reactant, i.e. generally more than 50 volume percent may be used.

The zeolite catalysts herein described are members of a novel class of zeolites exhibiting some unusual properties. These catalysts induce profound transformations of aliphatic hydrocarbons to aromatic hydrocarbons in commercially desirable yields and are generally highly effective in conversion reactions involving aromatic hydrocarbons. Although they have unusually low alumina contents, i.e. high silica to alumina ratios, they are very active even when the silica to alumina ratio exceeds 30. This activity is surprising since catalytic activity is generally attributed to framework aluminum atoms and cations associated with these aluminum atoms. These catalysts retain their crystallinity for long periods in spite of the presence of steam at high temperature which induces irreversible collapse of the framework of other zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits, when formed, may be removed by burning at higher than usual temperatures to restore activity. In many environments the zeolites of this class exhibit very low coke forming capability, conducive to very long times on stream between burning regenerations.

An important characteristic of the crystal structure of this class of zeolites is that it provides constrained access to, and egress from the intracrystalline free space by virtue of having a pore dimension greater than about 5 Angstroms and pore windows of about a size such as would be provided by 10-membered rings of oxygen atoms. It is to be understood, of course, that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline aluminosilicate, the oxygen atoms being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred type catalysts useful in this invention possess, in combination: a silica to alumina ratio of at least about 12; and a structure providing constrained access to the crystalline free space.

The silica to alumina ratio referred to may be determined by conventional analysis. This ratio is meant to represent, as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other form within the channels. Although catalysts with a silica to alumina ratio of at least 12 are useful, it is preferred to use catalysts having higher ratios of at least about 30. Such catalyst, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e. they exhibit "hydrophobic" properties. It is believed that this hydrophobic character is advantageous in the present invention.

The type zeolites useful in this invention freely sorb normal hexane and have a pore dimension greater than about 5 Angstroms. In addition, the structure must provide constrained access to larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings of oxygen atoms, then access by molecules of larger-cross section than normal hexane is excluded and the zeolite is not of the desired type. Windows of 10-membered rings are preferred, although, in some instances, excessive puckering or pore blockage may render these catalysts ineffective. Twelve-membered rings do not generally appear to offer sufficient constraint to produce the advantageous conversions, although puckered structures exist such as TMA offretite which is a known effective zeolite. Also, structures can be conceived, due to pore blockage or other cause, that may be operative.

Rather than attempt to judge from crystal structure whether or not a catalyst possesses the necessary constrained access, a simple determination of the "constraint index" may be made by passing continuously a mixture of an equal weight of normal hexane and 3-methylpentane over a small sample, approximately 1 gram or less, of catalyst at atmospheric pressure according to the following procedure. A sample of the catalyst, in the form of pellets or extrudate, is crushed to a particle size about that of coarse sand and mounted in a glass tube. Prior to testing, the catalyst is treated with a stream of air at 1000° F. for at least 15 minutes. The catalyst is then flushed with helium and the temperature adjusted between 550° F and 950° F. to give an overall conversion between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of catalyst per hour) over the catalyst with a helium dilution to give a helium to total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample of the effluent is taken and analyzed, most conveniently by gas chromatography, to determine the fraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Catalysts suitable for the present invention are those having a constraint index in the approximate range of 1 to 12. Constraint Index (CI) values for some typical catalysts are:

    ______________________________________                                         CAS                 C.I.                                                       ______________________________________                                         ZSM-5               8.3                                                        ZSM-11              8.7                                                        ZSM-12              2                                                          ZSM-38              2                                                          ZSM-35              4.5                                                        TMA Offretite       3.7                                                        Beta                0.6                                                        ZSM-4               0.5                                                        H-Zeolon            0.5                                                        REY                 0.4                                                        Amorphous Silica-Alumina                                                                           0.6                                                        Erionite            38                                                         ______________________________________                                    

It is to be realized that the above constraint index values typically characterize the specified zeolites but that such are the cumulative result of several variables used in determination and calculation thereof. Thus, for a given zeolite depending on the temperature employed within the aforenoted range of 550° to 950° F., with accompanying conversion between 10 and 60%, the constraint index may vary within the indicated approximate range of 1 to 12. Likewise, other variables such as the crystal size of the zeolite, the presence of possibly occluded contaminants and binders intimately combined with the zeolite may affect the constraint index. It will accordingly be understood by those skilled in the art that the constraint index, as utilized herein, while affording a highly useful means for characterizing the zeolites of interest is approximate, taking into consideration the manner of its determination, with the probability, in some instances, of compounding variable extremes. However, in all instances, at a temperature within the above-specified range of 550° to 950° F., the constraint index will have a value for any given zeolite of interest herein within the approximate range of 1 to 12.

The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38, and other similar materials. Recently issued U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated herein by reference.

ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, the entire contents of which are incorporated herein by reference.

ZSM-12 is more particularly described in U.S Pat. No. 3,832,449, the entire contents of which are incorporated herein by reference.

ZSM-38 is more particularly described in U.S. Application Ser. No. 528,060, filed Nov. 29, 1974. This zeolite can be identified, in terms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.3-2.5)R.sub.2 O : (0-0.8)M.sub.2 O : Al.sub.2 O.sub.3 : 8 SiO.sub.2

where R is an organic nitrogen-containing cation derived from a 2-(hydroxyalkyl) trialkylammonium compound and M is an alkali metal cation, and is characterized by a specified X-ray powder diffraction pattern.

In a preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.4-2.5)R.sub.2 O : (0-0.6) M.sub.2 O : Al.sub.2 O.sub.3 : xSiO.sub.2

wherein R is an organic nitrogen-containing cation derived from a 2-(hydroxyalkyl)trialkylammonium compound, wherein alkyl is methyl, ethyl or a combination thereof, M is an alkali metal, especially sodium, and x is from greater than 8 to about 50.

The synthetic ZSM-38 zeolite possesses a definite distinguishing crystalline structure whose X-ray diffraction pattern shows substantially the significant lines set forth in Table I. It is observed that this X-ray diffraction pattern (significant lines) is similar to that of natural ferrierite with a notable exception being that natural ferrierite patterns exhibit a significant line at 11.33A.

                  TABLE I                                                          ______________________________________                                         d(A)                 I/I.sub.o                                                 ______________________________________                                         9.8 ± 0.20        Strong                                                    9.1 ± 0.19        Medium                                                    8.0 ± 0.16        Weak                                                      7.1 ± 0.14        Medium                                                    6.7 ± 0.14        Medium                                                    6.0 ± 0.12        Weak                                                      4.37 ± 0.09       Weak                                                      4.23 ± 0.09       Weak                                                      4.01 ± 0.08       Very Strong                                               3.81 ± 0.08       Very Strong                                               3.69 ± 0.07       Medium                                                    3.57 ± 0.07       Very Strong                                               3.51 ± 0.07       Very Strong                                               3.34 ± 0.07       Medium                                                    3.17 ± 0.06       Strong                                                    3.08 ± 0.06       Medium                                                    3.00 ± 0.06       Weak                                                      2.92 ± 0.06       Medium                                                    2.73 ± 0.06       Weak                                                      2.66 ± 0.05       Weak                                                      2.60 ± 0.05       Weak                                                      2.49 ± 0.05       Weak                                                      ______________________________________                                    

A further characteristic of ZSM-38 is its sorptive capacity providing said zeolite to have increased capacity for 2-methylpentane (with respect to n-hexane sorption by the ratio n-hexane/2-methyl-pentane) when compared with a hydrogen form of natural ferrierite resulting from calcination of an ammonium exchanged form. The characteristic sorption ratio n-hexane/2-methylpentane for ZSM-38 (after calcination at 600° C.) is less than 10, whereas that ratio for the natural ferrierite is substantially greater than 10, for example, as high as 34 or higher.

Zeolite ZSM-38 can be suitably prepared by preparing a solution containing sources of an alkali metal oxide, preferably sodium oxide, an organic nitrogen-containing oxide, an oxide of aluminum, an oxide of silicon and water and having a composition, in terms of mole ratios of oxides, falling within the following ranges:

    ______________________________________                                         R+             Broad       Preferred                                           ______________________________________                                         R+ + M+        0.2-1.0     0.3-0.9                                             OH.sup.- /SiO.sub.2                                                                           0.05-0.5    0.07-0.49                                           H.sub.2 O/OH.sup.-                                                                            41-500      100-250                                             SiO.sub.2 /Al.sub.2 O.sub.3                                                                   8.8-200     12-60                                               ______________________________________                                    

wherein R is an organic nitrogen-containing cation derived from a 2-(hydroxyalkyl) trialkylammonium compound and M is an alkali metal ion, and maintaining the mixture until crystals of the zeolite are formed. (The quantity of OH is calculated only from the inorganic sources of alkali without any organic base contribution). Thereafter, the crystals are separated from the liquid and recovered. Typical reaction conditions consist of heating the foregoing reaction mixture to a temperature of from about 90° to about 400° C. for a period of time of from about 6 hours to about 100 days. A more preferred temperature range is from about 150° C. to about 400° C. with the amount of time at a temperature in such range being from about 6 hours to about 80 days.

The digestion of the gel particles is carried out until crystals form. The solid product is separated from the reaction medium, as by cooling the whole to room temperature, filtering and water washing. The crystalline product is thereafter dried, e.g. at 230° F. for from about 8 to 24 hours.

ZSM-35 is more particularly described in U.S. Application Ser. No. 528,061, filed November 29, 1974. This zeolite can be identified, in terms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.3-2.5)R.sub.2 O : (0-0.8)M.sub.2 O : Al.sub.2 O.sub.3 : > 8 SiO.sub.2

wherein R is an organic nitrogen-containing cation derived from ethylenediamine or pyrrolidine and M is an alkali metal cation, and is characterized by a specified X-ray powder diffraction pattern.

In a preferred synthesized form, the zeolite has a formula, in terms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.4-2.5)R.sub.2 O : (0.0.6) M.sub.2 O : Al.sub.2 O.sub.3 : xSiO.sub.2

wherein R is an organic nitrogen-containing cation derived from ethylenediamine or pyrrolidine, M is an alkali metal, especially sodium, and x is from greater than 8 to about 50.

The synthetic ZSM-35 zeolite possesses a definite distinguishing crystalline structure whose X-ray diffraction pattern shows substantially the significant lines set forth in table II. It is observed that this X-ray diffraction pattern (with respect to significant lines) is similar to that of natural ferrierite with a notable exception being that natural ferrierite patterns exhibit a significant line at 11.33A. Close examination of some individual samples of ZSM-35 may show a very weak line at 11.3-11.5A. This very weak line, however, is determined not to be a significant line for ZSM-35.

                  TABLE II                                                         ______________________________________                                         d(A)                 I/I.sub.o                                                 ______________________________________                                         9.6 ± 0.20        Very Strong -                                                                  Very Very Strong                                          7.10 ± 0.15       Medium                                                    6.98 ± 0.14       Medium                                                    6.64 ± 0.14       Medium                                                    5.78 ± 0.12       Weak                                                      5.68 ± 0.12       Weak                                                      4.97 ± 0.10       Weak                                                      4.58 ± 0.09       Weak                                                      3.99 ± 0.08       Strong                                                    3.94 ± 0.08       Medium Strong                                             3.85 ± 0.08       Medium                                                    3.78 ± 0.08       Strong                                                    3.74 ± 0.08       Weak                                                      3.66 ± 0.07       Medium                                                    3.54 ± 0.07       Very Strong                                               3.48 ± 0.07       Very Strong                                               3.39 ± 0.07       Weak                                                      3.32 ± 0.07       Weak Medium                                               3.14 ± 0.06       Weak Medium                                               2.90 ± 0.06       Weak                                                      2.85 ± 0.06       Weak                                                      2.71 ± 0.05       Weak                                                      2.65 ± 0.05       Weak                                                      2.62 ± 0.05       Weak                                                      2.58 ± 0.05       Weak                                                      2.54 ± 0.05       Weak                                                      2.48 ± 0.05       Weak                                                      ______________________________________                                    

A further characteristic of ZSM-35 is its sorptive capacity proving said zeolite to have increased capacity for 2-methylpentane (with respect to n-hexane sorption by the ratio n-hexane/2-methylpentane) when compared with a hydrogen form of natural ferrierite resulting from calcination of an ammonium exchanged form. The characteristic sorption ratio n-hexane/2-methylpentane for ZSM-35 (after calcination at 600° C.) is less than 10, whereas that ratio for the natural ferrierite is substantially greater than 10, for example, as high as 34 or higher.

Zeolite ZSM-35 can be suitably prepared by preparing a solution containing sources of an alkali metal oxide, preferably sodium oxide, an organic nitrogen-containing oxide, an oxide of aluminum, an oxide of silicon and water and having a composition, in terms of mole ratios of oxides, falling within the following ranges:

    ______________________________________                                         R+             Broad       Preferred                                           ______________________________________                                         R+ + M+        0.2-1.0     0.3-0.9                                             OH.sup.- /SiO.sub.2                                                                           0.05-0.5    0.07-0.49                                           H.sub.2 O/OH.sup.-                                                                            41-500      100-250                                             SiO.sub.2 /Al.sub.2 O.sub.3                                                                   8.8-200     12-60                                               ______________________________________                                    

wherein R is an organic nitrogen-containing cation derived from pyrrolidine or ethylenediamine and M is an alkali metal ion, and maintaining the mixture until crystals of the zeolite are formed. (The quantity of OH is calculated only from the inorganic sources of alkali without any organic base contribution). Thereafter, the crystals are separated from the liquid and recovered. Typical reaction conditions consist of heating the foregoing reaction mixture to a temperature of from about 90° to about 400° C. for a period of time of from about 6 hours to about 100 days. A more preferred temperature range is from about 150° to about 400° C. with the amount of time at a temperature in such range being from about 6 hours to about 80 days.

The digestion of the gel particles is carried out until crystals form. The solid product is separated from the reaction medium, as by cooling the whole to room temperature, filtering and water washing. The crystalline product is dried, e.g. at 230° F., for from about 8 to 24 hours.

The specific zeolites described, when prepared in the presence of organic cations, are catalytically inactive, possibly because the intracrystalline free space is occupied by organic cations from the forming solution. They may be activated by heating in an inert atmosphere at 1000° F. for one hour, for example, followed by base exchange with ammonium salts followed by calcination at 1000° F. in air. The presence of organic cations in the forming solution may not be absolutely essential to the formation of this type zeolite; however, the presence of these cations does appear to favor the formation of this special type of zeolite. More generally, it is desirable to activate this type catalyst by base exchange with ammonium salts followed by calcination in air at about 1000° F. for from about 15 minutes to about 24 hours.

Natural zeolites may sometimes be converted to this type zeolite catalyst by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, and clinoptilolite. The preferred crystalline aluminosilicates are ZSM-5, ZSM-11, ZSM-12, ZSM-38 and ZSM-35, with ZSM-5 particularly preferred.

The catalyst of this invention may be in the hydrogen from or they may be base exchanged or impregnated to contain ammonium or a metal cation complement. It is desirable to calcine the catalyst after base exchange. The metal cations that may be present include any of the cations of the metals of Group I through VIII of the Periodic Table. However, in the case of Group IA metals, the cation content should in no case be so large as to effectively inactivate the catalyst.

In a preferred aspect of this invention, the catalysts hereof are selected as those having a crystal framework density, in the dry hydrogen form, of not substantially below about 1.6 grams per cubic centimeter. It has been found that zeolites which satisfy all three of these criteria are most desired because they tend to maximize the production of gasoline boiling range hydrocarbon products. Therefore, the preferred catalysts of this invention are those having a constraint index as defined above of about 1 to about 12, a silica to alumina ratio of at least about 12 and a dried crystal density of not less than about 1.6 grams per cubic centimeter. The dry density for known stuctures may be calculated from the number of silicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g., on page 19 of the article on Zeolite Structure by W. M. Meir. This paper, the entire contents of which are incorporated herein by reference, is included in "Proceedings of the Conference on Molecular Sieves, London, April 1967", published by the Society of Chemical Industry, London, 1968. When the crystal structure in unknown, the crystal framework density may be determined by immersing the dry hydrogen form of the zeolite in an organic solvent which is not sorbed by the crystal. It is possible that the unusual sustained activity and stability of this class of zeolites is associated with its high crystal anionic framework density of not less than about 1.6 grams per cubic centimeter. This high density, of course, must be associated with a relatively small amount of free space within the crystal, which might be expected to result in more stable structures. This free space, however, is important as the locus of catalytic activity.

Crystal framework densities of some typical zeolites are:

    ______________________________________                                                        Void         Framework                                          Zeolite        Volume       Density                                            ______________________________________                                         Ferrierite     0.28 cc/cc   1.76 g/cc                                          Mordenite      .28          1.7                                                ZSM-5, -11     .29          1.79                                               Dachiardite    .32          1.72                                               L              .32          1.61                                               Clinoptilolite .34          1.71                                               Laumontite     .34          1.77                                               ZSM-4 (Omega)  .38          1.65                                               Heulandite     .39          1.69                                               P              .41          1.57                                               Offretite      .40          1.55                                               Levynite       .40          1.54                                               Erinonite      .35          1.51                                               Gmelinite      .44          1.46                                               Chabazite      .47          1.45                                               A              .5           1.3                                                Y              .48          1.27                                               ______________________________________                                    

When synthesized in the alkali metal form, the zeolite is conveniently converted to the hydrogen form, generally be intermediate formation of the ammonium form as a result of ammonium ion exchange and calcination of the ammonium form to yield the hydrogen form. In addition to the hydrogen form, other forms of the zeolite wherein the original alkali metal has been reduced to less than about 1.5 percent by weight may be used. Thus, the original alkali metal of the zeolite may be replaced by ion exchange with other suitable ions of Groups IB to VIII of the Periodic Table including, by way of example, nickel, zinc, calcium or rare earth metals.

The crystals of zeolite in a form substantially free of alkali metal, i.e. containing less than about 1.5 weight percent alkali metal and preferably having at least a portion of the original cations associated therewith replaced by hydrogen, are then contacted with a boron compound.

Representative boron-containing compounds include boric acid, trimethylborate, boron hydride, boron oxide, boron sulfide, butylboron dimethoxide, butylboronic acid, dimethylboric anhydride, hexamethylborazine, phenylboric acid, triethylborane, tetramethylammonium borohydride, triphenyl boron and allylborate.

Reaction of the zeolite with the boron compound is effected by contacting the zeolite with such compound. Where the treating boron compound is a liquid, such compound can be in solution in a solvent at the time contact with the zeolite is effected. Any solvent relatively inert with respect to the treating compound and the zeolite may be employed. Suitable solvents include water and aliphatic, aromatic or alcoholic liquids. Where the boron-containing compound is, for example, trimethylborate, a hydrocarbon solvent such as n-octane may be employed. The boron-containing compound may be used without a solvent, i.e., may be used as a neat liquid. Where the boron-containing compound is in the gaseous phase, such as where gaseous dibroane is employed, the treating compound can be used by itself or can be used in admixture with a gaseous diluent inert to the boron-containing compound and the zeolite such as nitrogen or helium or with an organic solvent, such as octane.

Prior to reacting the zeolite with the boron-containing compound, the zeolite may be dried. Drying can be effected in the presence of air. Elevated temperatures may be employed. However, the temperature should not be such that the crystal structure of the zeolite is destroyed.

Heating of the boron-containing catalyst subsequent to preparation and prior to use is also preferred. The heating can be carried out in the presence of oxygen, for example, air. Heating can be at a temperature of about 150° C. However, higher temperatures, i.e., up to about 500° C are preferred. Heating is generally carried out for 3-5 hours but may be extended to 24 hours or longer. While heating temperatures above about 500° C can be employed, they are not necessary. At temperatures of about 1000° C, the crystal structure of the zeolite tends to deteriorate.

The amount of boron incorporated with the zeolite should be at least about 0.2 percent by weight. However, it is preferred that the amount of boron in the zeolite be at least about 1 percent by weight when the same is combined with a binder, e.g. 35 weight percent of alumina. The amount of boron can be as high as about 20 percent by weight or more depending on the amount and type of binder present. Preferably, the amount of boron added to the zeolite is between about 1.5 and 10 percent by weight. Without being limited by any theoretical considerations, it is contemplated that boron is actually present in the zeolite in an oxidized state, such as B₂ O₃.

The amount of boron incorporated with the zeolite by reaction with a boron-containing compound will depend upon several factors. One of these is the reaction time, i.e. the time that the zeolite and the boron-containing source are maintained in contact with each other. With greater reaction times, all other factors being equal, a greater amount of boron is incorporated with the zeolite. Other factors upon which the amount of boron incorporated with the zeolite is dependent include reaction temperature, concentration of the treating compound in the reaction mixture, the degree to which the zeolite has been dried prior to reaction with the boron-containing compound, the conditions of drying of the zeolite after reaction of the zeolite with the treating compound, and the amount and type of binder incorporated with the zeolite.

In practicing the desired conversion process it may be desirable to incorporate the modified zeolite in another material resistant to the temperatures and other conditions employed in the conversion process. Such matrix materials include synthetic or naturally occurring substances as well as inorganic materials such as clay, silica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the modified zeolite include those of the montmorillonite and kaolin families, which families include the sub-bentonites and the kaolins commonly known as Dixie, McNamee-Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification.

In addition to the foregoing materials, the modified zeolites employed herein may be composited with a porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silicia-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may be in the form of a cogel. The relative proportions of finely divided modified zeolite and inorganic oxide gel matrix may vary widely with the zeolite content ranging from between about 1 to about 99 percent by weight and more usually in the range of about 5 to about 80 percent by weight of the composite.

The process of this invention is conducted such that conversion is carried out in the vapor phase by contact in a reaction zone, such as, for example, a fixed bed of catalyst, under effective conversion conditions, said catalyst being characterized as above-described and preferably hydrogen exchanged such that a predominate portion of is exchangeable cations are hydrogen ions. In general, it is contemplated that more than 50 percent and preferably more than 75 percent of the cationic sites of the crystalline aluminosilicate zeolite, above-described, will be occupied by hydrogen ions.

The conversion process described herein may be carried out as a batch-type, semi-continuous or continuous operation utilizing a fixed or moving bed catalyst system. One embodiment entails use of a fluidized catalyst zone wherein the hydrocarbon charge is passed concurrently or countercurrently through a moving fluidized bed of the catalyst. The fluidized catalyst after use is conducted to a regeneration zone wherein coke is burned from the catalyst in an oxygen-containing atmosphere, e.g. air, at an elevated temperature, after which the regenerated catalyst is recycled to the conversion zone for further contact with the toluene feed.

Reactivation of the boron-modified zeolite catalyst can be effected by passing a vaporized boron compound through the catalyst bed after the catalyst has been used for the desired conversion. Thus, for example, after a period of continued use of the catalyst, it can be revivified by passage therethrough of vaporized trimethylborate at an elevated temperature, i.e. above 250° C. over a 1/2 hot period of time. This treatment is then initially followed by heating in air at 150 cc/minute/gram of catalyst at about 550° C. for approximately 1/2 hour.

The following examples will serve to illustrate the process of the invention without limiting the same.

EXAMPLE 1

This example serves to illustrate disproportionation of toluene in the presence of a catalyst of HZSM-5 which has not been modified with boron.

A catalyst containing 65 weight percent acid ZSM-5 and 35 weight percent alumina was prepared as follows:

A sodium silicate solution was prepared by mixing 8440 lb. of sodium silicate (Q Brand -- 28.9 weight percent SiO₂, 8.9 weight percent Na₂ O and 62.2 weight percent H₂ O) and 586 gallons of water. After addition of 24 lb. of a dispersant of a sodium salt of polymerized substituted benzenoid alkyl sulfonic acid combined with an inert inorganic suspending agent (Daxad 27), the solution was cooled to approximately 55° F. An acid alum solution was prepared by dissolving 305 lb. aluminum sulfate (17.2 Al₂ O₃), 733 lb. sulfuric acid (93%) and 377 lb. sodium chloride in 602 gallons of water. The solutions were gelled in a mixing nozzle and discharged into a stirred autoclave. During this mixing operation, 1200 lb. of sodium chloride was added to the gel and thoroughly mixed in the vessel. The resulting gel was thoroughly agitated and heated to 200° F. in the closed vessel. After reducing agitation, an organic solution prepared by mixing 568 lb. tri-n-propylamine, 488 lb. n-propylbromide and 940 lb. methyl ethyl ketone was added to the gel. This mixture was reacted for 14 hours at a temperature of 200°-210° F. At the end of this period, agitation was increased and these conditions maintained until the crystallinity of the product reached at least 65% ZSM-5 as determined by X-ray diffraction. Temperature was then increased to 320° F. until crystallization was complete. The residual organics were flashed from the autoclave and the product slurry was cooled.

The product was washed by decantation using a flocculant of polyammonium bisulfate. The washed product containing less than 1% sodium was filtered and dried. The weight of dried zeolite was approximately 2300 lb.

The dried product was mixed with alpha alumina monohydrate and water (65% zeolite, 35% alumina binder on ignited basis) then extruded to form of 1/16 inch pellet with particle density < 0.98 gram/cc and crush strength of > 20 lb./linear inch.

After drying, the extruded pellets were calcined in nitrogen (700-1000 SCFM) for 3 hours at 1000° F., cooled and ambient air was passed through the bed for 5 hours. The pellets were then ammonium exchanged for one hour at ambient temperature (240 lb. ammonium nitrate dissolved in approximately 800 gallons of deionized water). The exchange was repeated and the pellets washed and dried. Sodium level in the exchanged pellets was less than 0.05 weight percent.

The dried pellets were calcined in a nitrogen-air mixture (10-12.5% air - 90-87.5% nitrogen) for 6 hours at 1000° F. and cooled in nitrogen alone.

This catalyst was used for disproportionating toluene by passing the same over 6.0 grams of the catalyst at a weight hourly space velocity of 3.5-3.6 at a temperature between 450° C. and 600° C. The conditions and results are summarized in Table III below.

                  TABLE III                                                        ______________________________________                                                       Tol.                                                             Temp,         Conv.    Selectivity, %                                                                             % Para in                                   ° C                                                                           WHSV    Mole %   Benzene                                                                               Xylenes                                                                               Xylene Product                            ______________________________________                                         450   3.6     7.4      50.7   46.5   24.7                                      500   3.5     20.5     44.6   53.8   24.5                                      550   3.5     38.8     48.0   48.8   24.2                                      600   3.5     49.2     54.4   41.7   24.1                                      ______________________________________                                    

It will be seen from the above results that the unmodified catalyst afforded a xylene product in which the para isomer was present in its normal equilibrium concentration of approximately 24 weight percent of the xylene fraction.

EXAMPLE 2

A catalyst of HZSM-5 modified to give 3.44 weight percent of boron was prepared by suspending 10 grams of large crystal (approximately 0.2 to 0.5 micron crystalline size) HZSM-5 in a solution of 5 grams of ortho boric acid (H₃ BO₃) in 50 milliliters of water at a temperature of 95°-97° C. for 17 hours. The supernatant liquid was decanted. The remaining solid was placed in an oven at 105°-200° C. and stirred frequently until it had a dry appearance. The dry product was then held at 200° C. for 1 hour. Two grams of this catalyst was used to test its ability to disproportionate toluene. The reaction conditions and results are summarized in Table IV below.

                                      TABLE IV                                     __________________________________________________________________________                                      Liq.                                          Run                                                                               Temp,                                                                              Hrs.   Tol.                                                                               Xylenes     Gas                                                                               Feed                                                                               Mole %    Aromatics                       No.                                                                               ° C                                                                         Run                                                                               WHSV                                                                               Conv.                                                                              p   m   o   cc cc/hr                                                                              Benzene                                                                             Xylene                                                                              C.sub.8.sup.+                   __________________________________________________________________________     1  500 1.0                                                                               3.8 4.4 29.4                                                                               50.5                                                                               20.1                                                                                0 8.8 50.4 49.6 0                               2  600 .7 "   20.8                                                                               28.2                                                                               50.8                                                                               20.9                                                                                10                                                                               "   51.8 47.7 .5                              3  650 1.2                                                                               "   24.5                                                                               34.9                                                                               48.0                                                                               17.2                                                                               130                                                                               "   55.5 43.8 .7                              4  700 1.0                                                                               "   20.2                                                                               73.0                                                                               21.1                                                                               5.9 295                                                                               "   62.6 35.8 1.6                                    CALCINE AIR 100 cc/min 600° C, 2 Hrs                             5  600 1.0                                                                               3.8 5.4 73.8                                                                               20.5                                                                               5.7  10                                                                               8.8 53.1 46.9 0                               6  650 1.0                                                                               "   9.7 74.2                                                                               20.2                                                                               5.6  50                                                                               "   56.3 43.0 .7                              7  700 1.0                                                                               "   13.1                                                                               87.2                                                                               10.1                                                                               2.7 290                                                                               "   63.8 34.4 1.9                             8  600 1.0                                                                               "   2.2 98  --  --   10                                                                               "   57.0 43.0 0                               __________________________________________________________________________

It will be seen from the above results that under comparable reaction conditions, the amount of para-xylene in the xylene fraction was considerably enriched as a result of the presence of boron in the catalyst comparison with the normal equilibrium concentration of approximately 24 weight percent of the xylene fraction.

EXAMPLE 3

In a manner similar to Example 2, the ammonium form of microcrystalline ZSM-5 (0.02 to 0.05 micron) was treated with aqueous ortho boric acid (H₃ BO₃) and tested for its ability to disproportionate toluene. The reaction conditions and results are summarized in Table V below.

                                      TABLE V                                      __________________________________________________________________________                                      Liq.                                          Run                                                                               Temp,                                                                              Hrs.   Tol.                                                                               Xylenes     Gas                                                                               Feed                                                                               Mole %    Aromatics                       No.                                                                               ° C                                                                         Run                                                                               WHSV                                                                               Conv.                                                                              p   m   o   cc cc/hr                                                                              Benzene                                                                             Xylene                                                                              C.sub.8.sup.+                   __________________________________________________________________________     1  500 1.0                                                                               3.8 2.3 96.7                                                                               3.3 0   10 8.8 51.2 48.8 0                               2  600 1.05                                                                              "   5.2 98.1                                                                               1.9 0   25 "   55.0 45.0 0                               3  650 1.0                                                                               "   3.1 95.2                                                                               3.7 1.7 20 "   70.8 28.7 .5                              4  700 1.0                                                                               "   1.7 56.5                                                                               43.5                                                                               0   170                                                                               "   90.2 4.5  5.3                             __________________________________________________________________________

EXAMPLE 4

Microcrystalline HZSM-5 in an amount of 15.26 grams was suspended in a solution of 2.61 grams of ortho boric acid (H₃ BO₃) in 40 ml. of water at a temperature of 75° C. After standing for approximately 17 hours, the entire slurry was poured into a crystallizing dish and placed in an oven at 115° C. The slurry was stirred every 5-10 minutes. After the bulk of the water had evaporated and a uniform solid was obtained, the temperature was slowly increased to 200° C. The weight of the solid was 16.46 grams. The catalyst was then thoroughly mixed and placed in an oven in an open dish, in air, at 500° C. for a period of 16 hours. Weight of the catalyst was 16.18 grams and its boron content was 2.7 weight percent. A 5 gram sample crushed to 10-14 mesh size and tested for its ability to disproportionate toluene. Reaction conditions and results are summarized in Table VI below.

                                      TABLE VI                                     __________________________________________________________________________                 Selectivity to Products, %                                         Run                                                                               Temp,                                                                              Toluene        Aromatics                                                                            Xylenes                                            No.                                                                               ° C                                                                         Conv, %                                                                             Benzene                                                                             Xylenes                                                                             C.sub.8.sup.+                                                                        p   m   o                                          __________________________________________________________________________     1  550 6.5  51.5 48.5 0     54.8                                                                               35.8                                                                               9.4                                        2  600 11.7 53.4 46.1 .5    72.1                                                                               23.0                                                                               4.9                                        3  650 11.3 60.2 38.6 1.2   87.6                                                                               11.0                                                                               1.4                                        __________________________________________________________________________

It will be evident from the above data that the amount of para-xylene produced was greatly in excess of its equilibrium concentration.

EXAMPLE 5

In a manner similar to Example 4, 20 grams of the ammonium form of microcrystalline ZSM-5 was suspended in a solution of 3.42 grams of ortho boric acid (H₃ BO₃) dissolved in 50 milliliters of water at a temperature of 77° C. After standing for 17 hours, the temperature was 90° C. The entire slurry was poured in a crystallizing dish, which was placed in an oven at approximately 110° C. The product was frequently stirred and when the bulk of the water had evaporated, the temperature was increased to 200° C. The catalyst was then placed in a furnace at 500° C. in air for a period of 16 hours. A 5 gram sample (13.4 cubic centimeters) of this catalyst was pressed into wafers, sized to 8-12 mesh and tested for its ability to disproportionate toluene. Conditions of reaction and results are summarized in Table VII below.

                                      TABLE VII                                    __________________________________________________________________________                 Selectivity to Products, %                                         Run                                                                               Temp,                                                                              Toluene        Aromatics                                                                            Xylenes                                            No.                                                                               ° C                                                                         Conv, %                                                                             Benzene                                                                             Xylenes                                                                             C.sub.8.sup.+                                                                        p   m   o                                          __________________________________________________________________________     1  550 9.2  52.0 47.8 .2    61.4                                                                               31.3                                                                               7.3                                        2  600 14.5 56.1 42.9 1.0   78.4                                                                               18.5                                                                               3.2                                        3  650 8.7  74.3 22.6 3.1   90.2                                                                               7.8 2.0                                        __________________________________________________________________________

It will again be seen that the amount of para-xylene produced was much greater than its normal equilibrium concentration.

EXAMPLE 6

In a manner similar to Example 4, 20 grams of microcrystalline ammonium ZSM-5 was suspended in a solution of 4.28 grams of ortho boric acid (H₃ BO₃) in 50 ml. of water at approximately 80° C. After standing overnight, the resulting slurry was transferred to a dish and water was evaporated with stirring while heating to 200° C. The catalyst was finally placed in an oven at 500° C. in air overnight to yield 20.99 grams of catalyst having a boron content of approximately 3.3 weight percent. A 5 gram (13.2 ml.) sample of 8-12 mesh particles was tested for its ability to disproportionate toluene. Conditions of reaction and results are summarized in Table VIII below.

                                      TABLE VIII                                   __________________________________________________________________________                 Selectivity to Products, %                                         Run                                                                               Temp,                                                                              Toluene        Aromatics                                                                            Xylenes                                            No.                                                                               ° C                                                                         Conv, %                                                                             Benzene                                                                             Xylenes                                                                             C.sub.8.sup.+                                                                        p   m   o                                          __________________________________________________________________________     1  550 4.8  47.4 52.3 0     91.8                                                                               7.2 1.0                                        2  600 7.2  51.4 48.3 .3    93.9                                                                               6.1 0                                          3  "   5.7  51.9 48.0 .2    95.2                                                                               4.8 0                                          4  "   4.7  52.2 47.6 .2    96.0                                                                               4.0 0                                          5  "   3.9  52.8 47.2 0     96.4                                                                               3.6 0                                          __________________________________________________________________________

It will be seen from the above results that very high selectivity to para-xylene in the xylene product was obtained utilizing this boron-modified zeolite catalyst.

EXAMPLE 7

In a manner similar to that described in Example 4, 20 grams of microcrystalline ZSM-5 was suspended in a solution of 6.69 grams of ortho boric acid (H₃ BO₃) in 40 ml. of water to yield 21.95 grams of modified catalyst, after the various heat treatments, with a calculated 4.92 weight percent boron. The catalyst so prepared was tested for its ability to disproportionate toluene. The conditions and results are summarized in Table IX below.

                                      TABLE IX                                     __________________________________________________________________________                     Selectivity to Products                                                        Mole %                                                         Run                                                                               Temp,   Toluene        Aromatics                                                                            Xylenes                                        No.                                                                               ° C                                                                         WHSV                                                                               Conv, %                                                                             Benzene                                                                             Xylenes                                                                             C.sub.8.sup.+                                                                        p   m   o                                      __________________________________________________________________________     1  600 3.4 2.6  58.0 42.0 0     99  --  --                                     2  "   "   8.5  53.4 46.6 0     93.8                                                                               6.2 0                                      3  "   "   8.2  53.6 46.1 .2    95.7                                                                               4.3 0                                      4  "   "   12.5 54.3 45.4 .2    92.7                                                                               6.4 .9                                     5  "   3.4 13.4 54.5 45.2 .2    90.8                                                                               7.7 1.5                                    6  "   1.7 19.5 55.6 43.3 1.1   84.3                                                                               13.0                                                                               2.7                                    7  "   0.7 27.9 58.5 39.8 1.6   71.3                                                                               23.3                                                                               5.4                                    8  "   3.4 9.1  54.1 45.5 .4    92.4                                                                               6.3 1.3                                    9  "   3.2 15.5 55.1 43.9 1.0   83.9                                                                               13.0                                                                               3.2                                    __________________________________________________________________________

In this case, after a number of runs of approximately one hour duration, the catalyst was treated with steam at 500° C. at a weight hourly space velocity at 4. The time treated after each run was as follows: after Run 1, 15 minutes; after Run 2, 10 minutes; after Run 3, 30 minutes; after Run 4, 30 minutes; after Run 8, 60 minutes. It is evident from the above data that after the first four treatments with steam that toluene conversion increased significantly from 2.6 to 13.4 percent, while the selectivity to para-xylene in the xylene product fraction remained above 90 percent. The conversion was increased further by reducing the space velocity, as shown in Runs 5-7, at the expense of some reduction in selectivity to para-xylene.

EXAMPLE 8

In a manner similar to that described in Example 4, 20 grams of microcrystalline ZSM-5 was suspended in a solution of 5.35 grams of ortho boric acid (H₃ BO₃) in 40 ml of water to give 21.03 grams of modified catalyst after the heat treatment with a calculated 4.06 weight percent boron. The resulting catalyst was tested for its ability to disproprotionate toluene. The conditions and results are summarized in Table X below.

                                      TABLE X                                      __________________________________________________________________________                     Selectivity to Products                                                        Mole %                                                         Run                                                                               Temp,   Toluene        Aromatics                                                                            Xylenes                                        No.                                                                               ° C                                                                         WHSV                                                                               Conv, %                                                                             Benzene                                                                             Toluene                                                                             C.sub.8.sup.+                                                                        p   m   o                                      __________________________________________________________________________     1  600 3.3 4.8  55.3 44.7 0     93.7                                                                               2.7 0                                      2  "   3.2 14.5 54.1 45.6 .2    87.2                                                                               11.1                                                                               1.7                                    3  "   1.7 19.9 55.6 43.8 .6    80.7                                                                               16.4                                                                               2.9                                    4  "   1.7 19.4 54.2 45.5 .3    79.1                                                                               18.1                                                                               2.8                                    5  "   "   18.2 54.9 44.7 .4    82.7                                                                               14.9                                                                               2.4                                    6  "   "   17.0 55.1 44.6 .3    84.6                                                                               13.2                                                                               2.2                                    7  "   "   16.1 54.6 45.0 .5    85.9                                                                               12.1                                                                               2.0                                    8  "   "   15.1 54.7 45.0 .3    86.9                                                                               11.2                                                                               1.9                                    9  "   "   14.3 54.2 45.5 .3    87.7                                                                               10.5                                                                               1.8                                    10 "   "   13.6 54.8 44.9 .3    88.3                                                                               10.0                                                                               1.7                                    11 "   "   13.1 54.0 45.7 .3    89.1                                                                               9.3 1.6                                    12 "   "   11.1 53.5 45.6 .9    90.3                                                                               8.3 1.4                                    13 "   "   10.0 54.7 45.0 .3    91.4                                                                               7.2 1.4                                    14 "   "   8.7  54.6 45.1 .3    92.5                                                                               6.2 1.2                                    15 "   "   7.0  55.3 44.4 .3    93.7                                                                               5.2 1.1                                    16 "   "   14.8 55.3 44.5 .2    85.8                                                                               12.4                                                                               1.9                                    __________________________________________________________________________

In these runs, the catalyst was activated with steam for a period of 30 minutes at a temperature of 500° C. at a weight hourly space velocity of 4 between Runs 1 and 2. The catalyst was calcined in air at 500° C. for 13/4 hours after Run 3. The catalyst was also calcined in air after Run 15 at 450° C. for 1/2 hour, then at 500° C. for 1/2 hour and finally at 550° C. for 13/4 hours. From the above results, it can be seen that the toluene conversion was significantly increased by activation with steam after Run 1. With Runs 4-15, conversion decreased steadily, probably due to coke formation and the selectivity to para-xylene in the xylene production fraction increased significantly. Catalyst activity was restored after Run 15 by calcination air.

EXAMPLE 9

With another sample of the catalyst described in Example 7, a test was made for its ability to disproportionate toluene. Reaction conditions and results are hereinafter summarized in Table XI. In this instance, the catalyst was activated with steam at a temperature of 500° C. for a period of 1/2 hour before use. After some initial runs with mixtures of toluene and hydrogen and toluene and nitrogen, the catalyst was treated with water at 500° C. for a period of one hour. It can be seen from Runs 1-7 that a toluene conversion of approximately 11-7 percent was realized while maintaining a selectivity to para-xylene in the xylene product of 88 to almost 94 percent. The catalyst was then calcined in air and activated with water between Runs 7 and 8 and between Runs 15 and 16. The overall effect of successive activation with water was to increase toluene conversion modestly and decrease the selectivity to para-xylene. With Runs 16-26, the effect of co-feeding hydrogen and nitrogen did not appear to be significant. Calcination with air regenerated the catalyst to give a level of conversion similar to that shown with the previous cycle. The above discussed results are shown in Table XI below

                                      TABLE XI                                     __________________________________________________________________________                                           Selectivity to Products                  Stream                                Mole %                                   Run                                                                               Time,     Temp,   Toluene                                                                             Xylenes               Aromatics                      No.                                                                               Hrs Feed  ° C                                                                         WHSV                                                                               Conv, %                                                                             p   m   o   Benzene                                                                             Toluene                                                                             C.sub.8.sup.+                  __________________________________________________________________________     1  39  Toluene                                                                              600 3.5 1.23 88.2                                                                               9.6 2.2 54.0 45.8 0.2                            2  40  "     "   "   10.8 90.35                                                                              7.88                                                                               1.77                                                                               52.9 46.7 .4                             3  41  "     "   "   9.5  92.0                                                                               6.6 1.4 53.4 46.4 0.2                            4  42  "     "   "   8.7  92.97                                                                              5.84                                                                               1.19                                                                               53.3 46.5 .2                             5  43  "     "   "   7.9  93.42                                                                              5.45                                                                               1.14                                                                               53.0 46.8 .2                             6  44  "     "   "   7.6  93.68                                                                              5.18                                                                               1.14                                                                               53.4 46.4 .2                             7  45  "     "   "   7.1  93.93                                                                              5.02                                                                               1.05                                                                               52.7 46.9 .4                                    CALCINE, AIR, 500° C, 100 cc/m, 16 HRS                                  ACTIVATE, H.sub.2 O, 500° C, 1 HR, WHSV=4                        8  45.5                                                                               Toluene                                                                              600 3.5 14.5 84.92                                                                              12.65                                                                              2.43                                                                               54.6 45.1 .3                             9  46  "     "   "   13.8 85.72                                                                              11.93                                                                              2.35                                                                               55.0 44.9 .1                             10 47  "     "   "   12.3 87.10                                                                              10.68                                                                              2.22                                                                               54.4 45.5 .1                             11 48  "     "   "   11.4 87.58                                                                              10.21                                                                              2.21                                                                               53.8 46.1 .1                             12 49  "     "   "   10.6 88.50                                                                              9.76                                                                               1.74                                                                               53.9 46.1 0                              13 50  "     "   "   10.2 88.31                                                                              9.62                                                                               2.07                                                                               53.3 46.6 .1                             14 51  "     "   "   9.6  88.50                                                                              9.44                                                                               2.06                                                                               53.3 46.5 .2                             15 52  "     "   "   9.2  88.64                                                                              9.26                                                                               2.10                                                                               53.1 46.8 0                                     CALCINE, AIR, 500° C, 100 cc/min, 3.5 HRS                               ACTIVATE H.sub.2 O, 19.8 cc/hr, 500° C, 1 HR                     16 53  Toluene                                                                              600 3.5 14.5 75.12                                                                              19.94                                                                              4.95                                                                               55.1 44.8 .1                             17 54  Toluene +                                                                            "   "   10.9 77.82                                                                              17.70                                                                              4.48                                                                               48.3 51.7 0                                     Hydrogen                                                                18 55  "     "   "   11.5 76.71                                                                              18.60                                                                              4.70                                                                               49.8 50.2 0                              19 56  "     "   "   11.4 76.79                                                                              18.51                                                                              4.70                                                                               49.8 50.2 0                              20 57  "     "   "   11.3 76.75                                                                              18.50                                                                              4.75                                                                               49.8 50.2 0                                     CALCINE, AIR, 100 cc/min, 500° C, 15.5 HRS                       21 58  Toluene                                                                              600 3.5 12.0 75.86                                                                              19.35                                                                              4.80                                                                               54.0 46.0                                                                              0                                22 59  Toluene +     10.6 77.09                                                                              18.21                                                                              4.70                                                                               50.1 49.7                                                                              .2                                      Nitrogen                                                                23 60  "             9.8  77.52                                                                              17.84                                                                              4.64                                                                               49.5 50.3                                                                              .2                               24 61  "             10.0 77.52                                                                              17.88                                                                              4.60                                                                               49.4 50.4                                                                              .2                               25 62  "             10.6 77.37                                                                              18.00                                                                              4.63                                                                               49.5 50.4                                                                              .1                               26 63  "             10.4 76.01                                                                              17.93                                                                              6.06                                                                               48.7 51.0                                                                              .3                                      CALCINE AIR, 500° C, 16 HRS, 100 cc/min                          27 64  Toluene                                                                              600 3.5 11.7 76.06                                                                              19.18                                                                              4.76                                                                               53.9 46.0 .1                             28 65  "     "   "   11.1 76.78                                                                              18.47                                                                              4.75                                                                               53.7 46.2 .1                             29 66  "     "   "   10.6 77.33                                                                              18.03                                                                              4.64                                                                               53.4 46.5 .1                             30 67  "     "   "   10.3 77.27                                                                              18.04                                                                              4.69                                                                               53.3 46.5 .2                             31 68  "     "   "   9.7  77.80                                                                              17.58                                                                              4.61                                                                               53.0 46.9 .1                             32 69  "     "   "   9.0  77.93                                                                              17.40                                                                              4.66                                                                               52.8 47.1 .1                             __________________________________________________________________________

EXAMPLE 10

The catalyst was prepared in a manner similar to that described in Example 8. Ten grams of the catalyst was tested for its ability to disproportionate toluene. Reaction conditions and results are summarized in Table XII below.

                                      TABLE XII                                    __________________________________________________________________________                                                   Mole %                           Run                                                                               Temp,            Tol.     Xylenes                     Other                 No.                                                                               ° C                                                                              WHSV    Conv.    Para    Meta Ortho                                                                              Benzene                                                                             Xylenes                                                                             Aromatics              __________________________________________________________________________     1  600      3.8     4.1      97.44    2.56                                                                               0   54.8 45.2 0                         Condition H.sub.2 O,                                                                    500°,                                                                           15 MIN,  WHSV = 2.04                                       2  "        "       6.2      96.27    3.73                                                                               0   53.0 47.0 0                         Condition H.sub.2 O,                                                                    500°,                                                                           30 MIN,  WHSV = 2.04                                       3  "        "       10.2     89.14    9.30                                                                               1.56                                                                               53.6 46.4 0                         Condition H.sub.2 O,                                                                    500°,                                                                           15 MIN,  WHSV = 2.05                                       4  "        "       11.4     83.48   13.60                                                                               2.92                                                                               53.5 46.2 .3                        COOL,    Remove Cat.,                                                                           Thoroughly Mix,                                                                         REPLACE                                           5  "        "       8.5      88.04    9.83                                                                               2.13                                                                               52.9 46.9 .2                        Condition H.sub.2 O,                                                                    500°,                                                                           15 MIN,  WHSV = 2.04                                       6  "        "       9.3      84.86   12.38                                                                               2.76                                                                               53.1 46.6 .3                     CONDITION H.sub.2 O, 500°, 30 MIN, WHSV = 2.04                          REMOVE CATALYST, THOROUGHLY MIX, REPLACE                                       7  600      3.8     11.9     74.24   20.43                                                                               5.33                                                                               53.4 46.4 .2                     8  "        "       12.0     73.60   20.9 5.49                                                                               53.2 46.6 .2                     9  "        "       11.2     74.40   20.25                                                                               5.35                                                                               53.8 45.9 .3                     10 "        "       11.0     75.17   19.72                                                                               5.11                                                                               53.0 46.6 .4                     11 "        "       10.3     75.87   19.10                                                                               5.03                                                                               53.2 46.4 .3                     12 "        "        9.6     76.44   18.71                                                                               4.85                                                                               53.3 46.4 .3                     13 "        "        9.1     77.06   18.18                                                                               4.76                                                                               53.3 46.3 .4                     14 "        "        8.7     77.42   17.86                                                                               4.72                                                                               53.2 46.4 .4                     15 "        "        8.4     77.45   17.80                                                                               4.75                                                                               53.0 46.6 .4                     16 "        "        8.0     78.03   17.32                                                                               4.65                                                                               53.2 46.5 .4                     __________________________________________________________________________

It will be seen from the above results that successive treatments with steam initially increased the conversion and reduced para-xylene selectivity. After Run 4, successive steam treatments did not appear to have an overall beneficial effect.

EXAMPLE 11

Ten grams of HZSM-5 having a crystal size of 0.2-0.5 micron were treated with a solution of 5 grams of H₃ BO₃ in 50 milliters of water at 95°-97° C. for a period of 17 hours. The supernatant liquid was decanted and the residue placed in a furnace at 105°-200° C. until a dry appearing solid was obtained. It was then held at 200° C. for 1 hour. A 2 gram sample of the resulting catalyst was tested for its ability to promote the alkylation of toluene with methanol. Reaction conditions and results are shown in Table XIII below.

                                      TABLE XIII                                   __________________________________________________________________________                    Feed                                                                           Toluene/                     Liq.                                                                              Mole %                          Run                                                                               Temp,                                                                              Hrs     Methanol                                                                               Toluene                                                                             Xylenes      Gas                                                                               Feed         Aromatics             No.                                                                               ° C                                                                         Run WHSV                                                                               Molar Ratio                                                                            Conv.                                                                               Para                                                                               Meta Ortho                                                                              cc cc/hr                                                                             Benzene                                                                             Xylenes                                                                             C.sub.8.sup.+         __________________________________________________________________________     1  600 1.0 3.8 4/1     13.6 96.9                                                                               2.4  6.8 170                                                                               8.8                                                                               14.0 84.4 1.6                   2  600 "   8.6 4/1     8.5  98.4                                                                               1.0   .6 520                                                                               19.8                                                                              2.9  94.6 2.5                   3  550 "   3.8 4/1     2.6  92.5                                                                               4.0  3.5 170                                                                               8.8                                                                               2.1  91.9 5.9                   4  600 "   "   2/1     2.5  92.3                                                                               4.0  3.7 330                                                                               8.8                                                                               3.4  91.5 4.8                          Calcine Air 100 cc/min, 600° C., 2 Hrs.                          5  600 1.0 3.8 2/1     25.6 77.9                                                                               18.3 3.9 250                                                                               8.8                                                                               14.1 82.3 3.7                   6  "   "   "   2/1     24.5 83.8                                                                               12.5 3.7 280                                                                               "  9.8  85.7 4.5                   7  "   "   "   2/1     22.2 87.8                                                                               8.6  3.6 325                                                                               "  7.3  87.8 4.9                   8  "   "   "   2/1     19.8 90.0                                                                               6.6  3.4 395                                                                               "  6.0  89.1 4.8                   9  "   "   "   2/1     17.3 92.0                                                                               5.0  2.9 450                                                                               "  4.9  90.8 4.4                   10 "   "   "   2/1     15.1 93.6                                                                               3.8  2.6 525                                                                               "  3.5  92.3 4.1                   __________________________________________________________________________

It will be seen from the above results that para-xylene was selectively produced as a result of alkylating toluene with methanol in the presence of the boron containing zeolite catalyst.

EXAMPLE 12

Propylene was passed over 5 grams (13.0 ml) of the catalyst described in Example 4 at a temperature of 400° C. utilizing a weight hourly space velocity of 2.6. Approximately 95 percent of the propylene was converted. Analysis showed the reaction product mixture to contain 8 percent aromatics, 39 percent C₁ -C₃ hydrocarbons and the remaining 53 percent C₄ and higher aliphatic compounds. Of the aromatic fraction, 30 percent was composed of xylenes, with 55.5 percent of the xylene fraction being para-xylene. It will be evident that the para-xylene isomer was present in a concentration significantly higher than the equilibrium value of 24 percent.

EXAMPLE 13

Propylene was passed over 2 grams of the catalyst described in Example 8 at a temperature of 400°-600° C. utilizing a weight hourly space velocity of 3.1 grams of propylene per gram of catalyst per hour. Reaction conditions and results are shown in Table XIV below:

                                      TABLE XIV                                    __________________________________________________________________________     Run         Propylene                                                                            Para-Xylene                                                                            Xylene In                                                                             Aromatic                                                                              Percent                                                                             Percent                           No.                                                                               ° C.                                                                        (Hrs.)                                                                              Conversion                                                                           In Xylenes                                                                             Aromatics                                                                             Selectivity                                                                           Ethylene                                                                            Olefin                            __________________________________________________________________________     1  400 1.0  95.9  78.3    27.1   22.0    1.3 13.9                              2  500 2.25 85.3  87.7    38.5   19.8    6.4 26.2                              3  600 4.0  62.6  86.6    26.6   23.7   27.1 53.3                              4  600 6.0  64.0  87.5    27.6   26.6   25.2 49.9                              5  600 8    61.4  87.2    28.4   23.4   26.3 53.3                              6  600 10   59.1  91.0    19.8   20.5   25.1 55.6                              7  600 25   35.4  91.0    11.4   8.9    20.1 60.1                                     Calcine Air 500° C., 0.5 Hour; 600° C., 3.5 Hours         8  400 1.3  94.9  77.8    26.6   12.8    1.6 18.2                              9  500 3.0  81.0  90.0    31.6   14.3    8.4 33.7                              10 600 5.6  58.7  87.1    16.7   18.9   25.7 58.0                              __________________________________________________________________________

From the above results, it will be seen that para-xylene was very selectively produced.

It is to be understood that the foregoing description is merely illustrative of preferred embodiments of the invention of which many variations may be made by those skilled in the art within the scope of the following claims without departing from the spirit thereof. 

I claim:
 1. A process for the selective production of para-xylene which comprises contacting toluene, under methylation conditions, with a methylating agent in the presence of a catalyst consisting essentially of a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12, said zeolite having combined therewith boron in an amount of at least about 0.2 percent by weight as a result of reaction of said zeolite with a boron-containing compound selected from the group consisting of boric acid, trimethylborate, boron hydride, boron oxide, boron sulfide, butylboron dimethoxide, butylboronic acid, dimethylboric anhydride, hexamethylborazine, phenylboric acid, triethylborane, tetramethylammonium borohydride, triphenyl boron and allylborate; cooling the resulting product and separating para-xylene therefrom.
 2. The process of claim 1 wherein said methylating agent is methanol.
 3. The process of claim 1 wherein said methylation conditions include a temperature between about 250° and about 750° C., a pressure between about 1 atmosphere and about 1000 psig utilizing a weight hourly space velocity of between about 1 and about
 2000. 4. The process of claim 1 wherein the molar ratio of methylating agent to toluene is between about 0.05 and about
 5. 5. The process of claim 3 wherein said temperature is between about 400° and about 600° C. and said liquid hourly space velocity is between about 5 and about
 1500. 6. The process of claim 1 wherein the boron content is between about 1 and about 20 weight percent.
 7. The process of claim 1 wherein said crystalline aluminosilicate zeolite is ZSM-5.
 8. The process of claim 1 wherein said crystalline aluminosilicate zeolite is ZSM-11.
 9. The process of claim 7 wherein ZSM-5 is predominantly in the hydrogen form.
 10. The process of claim 7 wherein ZSM-5 is present in combination with a binder therefor.
 11. The process of claim 1 wherein the source of boron is boric acid.
 12. The process of claim 1 wherein said zeolite has a silica to alumina ratio of at least about
 30. 13. The process of claim 1 wherein the boron content is between about 1.5 and about 10 weight percent. 